1. Field of the Invention
The present invention relates to an idling rpm control device for an internal combustion engine, particularly an automotive engine, and more particularly to a device for decreasing variations in rpm of an engine.
2. Description of the Prior Art
Recently, it has been necessary to control idling revolutions per minute (rpm) of engines precisely in order to improve the purification of exhaust gases and to decrease the fuel consumption. For this purpose, a method has been proposed which utilizes an arithmetic circuit including a micro computer or the like, which compares actual idling rpm with values previously stored in a memory of the arithmetic circuit to produce a signal for decreasing or increasing air supply when the actual rpm is higher or lower than the stored value, thereby controlling the air supply and hence the rpm of the engine in such a feedback control manner. As an amount of fuel supply (for example amount of fuel injection) depends upon an amount of air supply, a control of the amount of air supply results in a control of engine rpm.
With hitherto used control devices such as above described, however, it is impossible to eliminate periodic variations in rpm of engines, although it is possible to control average rpm to desired values.
The periodic variation in rpm is partly caused by irregular combustion in an engine. In other words, in the event that a mixture is "rich" or "lean" or a fuel-air ratio of the mixture is higher or lower than a desired fuel-air ratio, the combustion of the mixture in the engine becomes unstable with resulting variations in rpm.
With the hitherto used idling an rpm control device as above described, since only an amount of air supply is feedback controlled and an amount of fuel previously determined corresponding to the amount of air supply is supplied, there is a tendency for variations in rpm to occur due to irregular combustion, with the result that passengers often feel uncomfortable.